EXTERNAL FORCES AND PLANTS 39 



These experiments must not be interpreted to mean that the 

 roots somehow know that there is more water on one side, or 

 that they have any way of choosing to go toward the water. 

 .We may say merely that the plants are influenced in their 

 behavior by these various external conditions. 



60. Fitness. In the three sets of responses studied namely, 

 the responses to gravity, to light, and to water we can see an 

 advantage to the plant in behaving as it does. The tendency of 

 the root to grow downward will, on the whole, bring the roots 

 of the plants into the soil, where the conditions for getting 

 water are more favorable than they are on the surface of the 

 soil. We can see that the responses of the shoot to gravity 

 and light are, in the long run, likely to bring the plant into 

 situations favorable to its further development. But it does 

 not follow that everything that the plant does is of advantage. 



We saw that light actually interferes with the growth of the 

 plant, and yet, on the whole, the plant turns toward the light. 

 Is not this response injurious and suicidal ? But we shall find 

 later (p. 73) that the light is of great importance in the life 

 of the plant in ways connected not with growth but with the 

 making of food. 



Many of the responses of animals even of higher animals, 

 including ourselves are just as mechanical as some of these 

 simpler responses of plants. They are mechanical in the sense 

 that they result from the structure of the organism, and do 

 not involve anything in the nature of thinking or desiring or 

 choosing. Like the responses of the plants, most of the ani- 

 mal responses that we are likely to notice are of a kind that 

 help the organism in keeping alive for example, by prevent- 

 ing injury or by helping in the obtaining of food. But among 

 animals, as among plants, we can find responses that seem to 

 be of no value whatever to the life of the organism, and some 

 that are even injurious under certain circumstances. 



